The invention relates to a method for producing a hot strip steel in which following finish rolling, the hot strip is subjected to a cooling process carried out in several stages.
Cooling of a hot strip following finish rolling, which normally takes place in several passes, is very important as far as the characteristic properties of the materials of the strip are concerned. Among other things, the application of suitable cooling makes it possible to influence the microstructure itself, as well as the individual types of structure which make up this microstructure. It is thus possible for example, by way of the cooling process, to influence the strength, toughness and hardness of a hot strip.
The article “Hot rolled coils for special applications”, A. De Vito et al., BTF—special issue 1986, pages 137-141, describes various studies which document the influence which cooling has on the production of hot strip. These studies show that it is for example advantageous in the production of a dual-phase hot strip steel (DP hot strip steel) to carry out the cooling process which follows finish rolling, in three stages. In the first and last of these three stages, the strip passes through conventional laminar cooling sections, arranged so as to be spaced apart from each other, whereby coolant is sprayed onto the strip in the form of a multitude of “veils”, one arranged behind the other in the direction of conveyance of the strip. The cooling rate achieved in this process is approx. 70° C./s in the first cooling stage. Cooling of the strip in the third stage takes place more slowly than in the first stage.
In the intermediate stage which is passed between the laminar cooling sections, in the known method, cooling takes place by exposure to air, whereby again the cooling rate achieved in this stage is far below that of the last stage of the cooling process.
It has been shown that using the previously explained, known method, DP hot strip steels can be produced which do not contain molybdenum, with said DP steels comprising distinct martensite and ferrite constituents. The respective hot strip steels are of increased strength and toughness.
At the same time however, a decrease in ductility has to be accepted. Furthermore, it has become evident that the improvements which can be achieved with the known method are not sufficient to meet the requirements prescribed of hot strip produced in this way, in particular they do not meet hardness requirements.